This invention relates to a ground fault interrupter (GFI) and more particularly to such a GFI capable of using a single differential transformer core and capable of actuating a circuit breaker when either a hot-line to ground fault (H-GF) or a neutral-line to ground fault (N-GF) occurs.
Ground fault interrupters may have the features illustrated in FIG. 1. A hot line 10 and a neutral line 11 connect an electrical load 13 to an AC power source 14 at terminals 14a and 14b through the switches 15 and 16 of an electromechanical breaker 17. The breaker 17 includes an actuating solenoid 18.
The power lines 10 and 11 are threaded through a toroidal magnetic core 2-having a multi-turn secondary winding 21. The secondary winding 21 is connected to a ground fault detector circuit 22 that receives electrical energy from lines 10 and 11 through a full wave rectifier bridge circuit 23 including diodes 24, 25, 26 and 27 and resistor 28.
Resistor 30 represents a hot line to ground fault (H-GF) while resistor 31 represents a neutral line to ground fault (N-GF). Resistor 30 may further represent a human body, for example one standing on a damp floor and touching the metal housing of an electrical appliance (not shown) that is shorted to the hot line. When a hot line fault (H-GF) 30 exists, some of the current in hot line 10 does not return to the source 14 via neutral line 11 but rather is diverted around the core 20 through ground 33 to source 14. In this event the excess of current in line 10 over that in line 11 generates a magnetic field in core 20 and induces a hot line to ground fault voltage at secondary winding 21, that is proportional to the fault current in resistor 30. If this voltage exceeds a predetermined safe limit, the detector 22 trips the breaker 17 and removes the load 13 from the AC source. It is customary to use a differential transformer having a secondary winding 21 having hundreds of turns so that the primary to secondary voltage step up ratio makes fault detection simpler. This requires the use of a stable core (20) material, usually an iron-nickel alloy, leading to a relatively high cost transformer. When detecting circuits are largely integrated, the transformer represents a major cost element in the G.F.I.
If a N-GF 31 also exists, some of the fault current flowing through H-GF 30 may be returned to neutral line 11 thus reducing the H-GF induced voltage at winding 21 and rendering the H-GF detection system ineffective. Thus such a fault (N-GF) is not itself a hazard but potentially disables the hot fault (H-GF) detector.
H-GF currents in excess of 6 milliamperes are considered potentially lethal and a class A GFI as defined by Underwriters Laboratories is required to trip the breaker 17 within a time T for a fault current I between 6 and 264 milliamperes, where ##EQU1## This amounts to 5.6 seconds and 30 milliseconds (about two cycles of a 60 Hz line) corresponding to the 6 and 264 milliamperes, respectively. Neutral line to ground faults of less than 2 ohms are considered by Underwriters Laboratories as being capable of desensitizing an imbalanced-load-current type fault detector system as described above to an unacceptable level.
The requirement for fast tripping in response to a H-GF or a N-GF is especially difficult to meet, considering the relatively high load currents that may flow in power lines 10 and 11 and considering the not infrequent presence on these lines 10 and 11 of large amplitude impulse high frequency noise signals. Such noise tends to interfere with the sensitive detector circuits and then often trips the breaker when no fault exists at all. The high voltage step-up ratio of conventional GFI differential transformers that makes fault detection simpler, also makes nuisance tripping due to noise more difficult to avoid. The H-GF current I.sub.30 produces a voltage at the H-GF voltage detector of ##EQU2## where N.sub.2 /N.sub.1 is the winding step-up ratio, Lp is the primary inductance (e.g. of winding 10) and I.sub.30 is the peak fault current at line frequency f, usually 50 Hz or 60 Hz. With such a system, noise current spikes, containing much higher-frequency energy, are greatly magnified through the transformer.
A large number of secondary turns tends to increase the interwinding capacitance between primary lines 10 and 11 and secondary winding 21, which more freely transmits line voltage spikes to the sensitive H-GF detector.
It is an object of the present invention to provide a low cost but reliable and fast responding ground fault interrupter.
It is another object of this invention to provide a GFI having a H-GF current amplifier that presents an extremely low input impedance so that the differential transformer serves as a current transformer.
It is a further object of this invention to provide a ground fault interrupter capable of employing a single low cost non-critical differential transformer.
It is yet another object of this invention to provide a GFI having a N-GF detector that samples a N-GF related voltage a plurality of times during a portion of every cycle of the power line frequency and provides a signal proportional to the average value of the plurality of samples so as to be substantially insensitive to noise.